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Langmuir 2005, 21, 11480-11483
Influence of a Water Rinse on the Structure and Properties of Poly(3,4-ethylene dioxythiophene):Poly(styrene sulfonate) Films Dean M. DeLongchamp,*,† Bryan D. Vogt,*,† Charles M. Brooks,† Kenji Kano,† Jan Obrzut,† Curt A. Richter,‡ Oleg A. Kirillov,‡ and Eric K. Lin† Polymers Division and Semiconductor Electronics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 Received May 27, 2005. In Final Form: September 15, 2005 Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS) films exhibit a complex structure of interconnected conductive PEDOT domains in an insulating PSS matrix that controls their electrical properties. This structure is modified by a water rinse, which removes PSS with negligible PEDOT loss. Upon PSS removal, film thickness is reduced by 35%, conductivity is increased by 50%, and a prominent dielectric relaxation is eliminated. These results suggest that the removed PSS is not associated with PEDOT and that the conductive domain network is not substantially altered by the removal of a significant fraction of insulator. The removal of PSS may benefit organic light emitting diode fabrication by reducing acid attack on indium tin oxide electrodes and lead to more robust performance in switching circuits by extending the working frequency range.
Introduction The use of conducting polymer dispersions is rapidly increasing because of increases in their conductivity and stability over the past decade. Poly(3,4-ethylene dioxythiophene) (PEDOT),1-3 in particular, is used in a wide range of functional applications because cast films are both electronically conductive and optically transparent. These characteristics are required in many applications including antistatic coatings,4 flexible electronic interconnects,5,6 electrochromic windows,7-9 and hole transport layers in organic light emitting diodes (OLEDs).10-12 PEDOT is typically applied in a colloidal form consisting of oxidized PEDOT chains electrostatically associated with a poly(styrene sulfonic acid) (PSS) dopant, with an excess of PSS added to stabilize the dispersion. Solid films cast from this colloidal dispersion exhibit a complex threedimensional distribution of PEDOT and PSS, a morphology that controls the film electrical properties. Despite the widespread use of PEDOT:PSS, the correlation of PEDOT:PSS film morphology to its electrical properties remains unclear. Factors that influence the morphology * To whom correspondence may be addressed. E-mail:
[email protected] (D.M.D.);
[email protected] (B.D.V.). † Polymers Division, NIST. ‡ Semiconductor Electronics Division, NIST. (1) Heywang, G.; Jonas, F. Adv. Mater. 1992, 4, 116. (2) Groenendaal, B. L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. Adv. Mater. 2000, 12, 481. (3) Wessling, B. Synth. Met. 2003, 135, 265. (4) Jonas, F.; Morrison, J. T. Synth. Met. 1997, 85, 1397. (5) Zhang, F. L.; Nyberg, T.; Ingana¨s, O. Nano Lett. 2002, 2, 1373. (6) Hohnholz, D.; Okuzaki, H.; Macdiarmid, A. G. Adv. Funct. Mater. 2005, 15, 51. (7) Pei, Q.; Zuccarello, G.; Ahlskog, M.; Ingana¨s, O. Polymer 1994, 35, 1347. (8) Sapp, S. A.; Sotzing, G. A.; Reddinger, J. L.; Reynolds, J. R. Adv. Mater. 1996, 8, 808. (9) Argun, A. A.; Cirpan, A.; Reynolds, J. R. Adv. Mater. 2003, 15, 1338. (10) Carter, S. A.; Angelopoulos, M.; Karg, S.; Brock, P. J.; Scott, J. C. Appl. Phys. Lett. 1997, 70, 2067. (11) Kim, J. S.; Granstrom, M.; Friend, R. H.; Johansson, N.; Salaneck, W. R.; Daik, R.; Feast, W. J.; Cacialli, F. J. Appl. Phys. 1998, 84, 6859. (12) Makinen, A. J.; Hill, I. G.; Shashidhar, R.; Nikolov, N.; Kafafi, Z. H. Appl. Phys. Lett. 2001, 79, 557.
include the dispersion composition and processing such as thermal annealing or solvent treatments. Previous investigations into the morphology of PEDOT: PSS films using scanning tunneling microscopy revealed an inhomogeneous three-dimensional network of highly conductive PEDOT islands dispersed in an insulating PSS matrix.13,14 These studies and others employing deuterated PSS with neutron reflectivity15,16 have found a top PSSrich layer in spin-cast PEDOT:PSS films. Although excess PSS is needed to stabilize the dispersion, these results suggest that the final PEDOT:PSS films contain substantial amounts of PSS that segregates from the PEDOT: PSS complex. Because PSS is an electrical insulator, an excess of PSS could limit the film conductivity. Further, PSS is known to degrade the performance of OLEDs over time by its acid attack on indium tin oxide (ITO) anodes.15 Studies of the structure and state of unassociated PSS and its role in the development of film morphology could provide new strategies for the optimization of PEDOT: PSS electrical properties. Here, we investigate the influence of a water rinse on the structural and electrical properties of spin-cast PEDOT:PSS films. We begin with films spin-cast from the most commonly used formulation with a PEDOT to PSS mass ratio of 1:2.5. The films are dried and then washed with water. X-ray reflectivity (XR) and visible-nearinfrared (vis-NIR) spectroscopy are used to quantify changes in the film thickness and composition. Direct current and frequency-based electrical measurements are used to probe the electronic conductivity and dielectric relaxations of these films. These measurements provide information about the heterogeneity of the PEDOT:PSS film structure and the association of PSS with PEDOT. (13) Kemerink, M.; Timpanaro, S.; De Kok, M. M.; Meulenkamp, E. A.; Touwslager, F. J. J. Phys. Chem. B 2004, 108, 18820. (14) Timpanaro, S.; Kemerink, M.; Touwslager, F. J.; De Kok, M. M.; Schrader, S. Chem. Phys. Lett. 2004, 394, 339. (15) Higgins, A. M.; Martin, S. J.; Jukes, P. C.; Geoghegan, M.; Jones, R. A. L.; Langridge, S.; Cubitt, R.; Kirchmeyer, S.; Wehrum, A.; Grizzi, I. J. Mater. Chem. 2003, 13, 2814. (16) Jukes, P. C.; Martin, S. J.; Higgins, A. M.; Geoghegan, M.; Jones, R. A. L.; Langridge, S.; Wehrum, A.; Kirchmeyer, S. Adv. Mater. 2004, 16, 807.
10.1021/la051403l CCC: $30.25 © 2005 American Chemical Society Published on Web 10/28/2005
Structure and Properties of PEDOT:PSS Films
Langmuir, Vol. 21, No. 24, 2005 11481
Experimental Section [Certain equipment, instruments, or materials are identified in this paper in order to adequately specify the experimental details. Such identification does not imply recommendation by the National Institute of Standards and Technology nor does it imply the materials are necessarily the best available for the purpose.] Materials. The PEDOT:PSS (Baytron P) dispersion was obtained from H. C. Starck. Dilutions were made with MilliQ (Millipore) filtered deionized water. Dispersions were cast upon borosilicate float glass (Swift Glass) using a Headway spinprocessor with a two step process of (1) 500 rpm/s to 1000 rpm for 30 s and (2) 500 rpm/s to 2000 rpm for 90 s. Structural Characterization. XR was performed on a modified Scintag X-ray diffractometer. Instrumental details are provided elsewhere.17 The reflectivity profiles were fit using the Parratt formalism18 following the least-squares algorithm of Ankner and Majkrzak.19 Composition analysis was performed using vis-NIR spectroscopy. Spectra were collected using a Perkin-Elmer Lambda 9 Spectrophotometer, and data were analyzed using the Beer-Lambert law. Electrical Characterization. Electronic measurements were performed using custom test beds. Conductivity test beds were fabricated using negative tone photolithography to define gold features consisting of 1000 Å gold and a 100 Å chromium adhesion layer. Impedance measurements required thermal evaporation of Au atop cast films at a base pressure of 10-6 Torr and a deposition rate of 0.5 Å/s. Conductivity measurements were performed using a Keithley 6430 Source-Measurement Unit, and impedance measurements were performed using an HP 4294A impedance analyzer. Conductivity measurements were performed inside a Cascade Microtech Microchamber probe station to minimize electrical noise.
Results and Discussion The influence of a water rinse on the structure and properties of PEDOT:PSS films was evaluated by measuring the thickness, composition (via vis-NIR absorbance), and electrical properties of unwashed and washed films cast from dilutions of the stock dispersion. The water rinse procedure involved three steps. First, PEDOT:PSS films were spin-cast atop borosilicate float glass substrates. Second, the films were dried at 110 °C under vacuum for 24 h. Third, neutral deionized water was spin-cast upon the dried films at 2000 rpm, resulting in “washed” films. The washed films were re-dried at 110 °C under vacuum. These films on borosilicate float glass were used for both XR measurements and vis-NIR spectroscopy. Gold bottom-contact electrode structures were used to measure conductivity and perform impedance spectroscopy on films with casting and washing conditions identical to those on borosilicate float glass. Specular XR was employed to measure plane-averaged film thickness and roughness before and after washing. A representative XR dataset is shown in Figure 1a; the logarithm of the reflected intensity is shown as a function of the scattering vector q ()(4π/λ) sin θ, where θ is the incident angle of reflection). The constructive and destructive interference fringes are fit with a model profile composed of a single polymer layer atop borosilicate float glass. The real space profiles in Figure 1b represent the nonlinear least-squares best fits to the reflectivity dataset shown in Figure 1a, where the thickness, roughness, and X-ray scattering length density (Qc2) of the polymer layer were allowed to vary. Water washing reduces the thickness of the film in Figure 1 from 65 to 42 nm. This thickness reduction of (17) Wallace, W. E.; Vanzanten, J. H.; Wu, W. L. Phys. Rev. E 1995, 52, R3329. (18) Parratt, L. G. Phys. Rev. 1954, 95, 359. (19) Ankner, J. F.; Majkrzak, C. F. Proc. SPIE 1992, 1738, 260.
Figure 1. (a) XR for films spin-cast atop borosilicate float glass from a 3:2 dilution of PEDOT:PSS stock to water. The solid lines correspond to the best fit of the data; traces are offset for clarity. (b) Real space X-ray scattering length density profile for unwashed and washed films corresponding to best fits of the traces in a. Table 1. Film Thickness Loss and PEDOT Loss with Water Washinga stock (%)
untreated thickness |(nm)
treated thickness (nm)
thickness loss
PEDOT loss
0.33 0.50 0.66 0.83 1.00
16.3 32.9 65.0 97.2 147.0
11.6 20.4 41.8 56.8 104.7
29% 38% 36% 42% 29%
4.5% 4.9% 5.5% 4.2% 1.9%
a The uncertainty of thickness measurements is
